: The human T-cell leukemia virus, type I, is causally linked to a fatal lymphoproliferative disease called adult T-cell leukemia (ATL). The virally-encoded Tax protein appears directly responsible for the creation of an intracellular environment that is permissive to malignant transformation. Tax is critical to the viral life cycle, as it activates HTLV-I transcription following the assembly of multiprotein complexes in the transcriptional control region of the virus. Recent evidence indicates that the Tax-containing promoter complex is responsible for recruitment of the cellular coactivators, CBP/p300, with the subsequent strong activation of viral transcription. CBP and p300 are highly related proteins that belong to a novel class of transcriptional activators that possess histone acetyltransferase (HAT) activity, and appear to play a significant role in gene-specific activation though modulation of chromatin structure. CBP and p300 are both highly pleiotropic proteins that function in virtually all known cellular programs, including development, differentiation, and cell death. Because of their central role in fundamental biological processes, dysregulation of these proteins has been strongly correlated with human disease, including hematologic malignancies. Based on these observations, we hypothesize that the strong Tax dependence on CBP/p300 underlies HTLV-I associated neoplastic transformation of the infected T-cell. Currently, very little is known about the molecular mechanisms of CBP/p300 transcription function. In this application, we propose to investigate the role of CBP/p300 in Tax-mediated transcriptional activation in vitro. We have selected this system, as the Tax-CBP/p300 interaction is among the best characterized of the coactivator-transcription factor interactions to date. We plan to explore CBP-mediated Tax transcriptional activation using chromatin-assembled templates in vitro. Our methods will allow biochemical dissection of the TaxCBP/p300 interactions, and enable evaluation of putative nucleosome and transcription factor modifications that accompany transcriptional activation. Outcomes of the proposed studies may reveal basic aspects of Tax and CBP/p300 function as they pertain to gene regulation, chromatin structure, and leukemogenesis.